1. Field of the Invention
A sensor for detecting small changes in temperature for any differential temperature measurement application is disclosed. Specifically, a microfabricated thermopile is optimized as a differential temperature sensor. More specifically, an improved thermopile sensor for microcalorimetric applications is fabricated from silicon that is capable of resolving a 10.sup.-6 .degree. C. change with a range of background temperatures between about -50.degree. C. and 100.degree. C.
2. Description of the Background Art
Calorimetry is a powerful and versatile analytical tool because nearly all physical and chemical changes involve the evolution or absorption of heat. However, the high cost of presently available calorimeters, and their relatively low sample throughput, have prevented calorimetry from taking its place as an alternative to analytical methods presently used which are often slower or otherwise less satisfactory. One limiting factor in producing a viable and low cost calorimeter has been the inadequacy of sensors for detecting changes in temperature between a sample and a reference. The subject invention overcomes this and related problems.
A pair of junctions of dissimilar electrically conducting materials, where one is held at a higher temperature than the other, produces a measurable voltage. This phenomenon, observed in semiconductors and metals, was described by Seebeck at the end of the 19th century. With semiconductor couples, the voltage depends on the dopant concentration, which also affects the resistivity of the semiconductor. For a silicon p-n thermocouple junction with resistivity of 0.01 .OMEGA.cm for both n and p-types, the Seebeck coefficient, .DELTA.V/.DELTA.T, is approximately 500 .mu.V/.degree.C. By connecting hot and cold couples together in a serial fashion, one can increase the voltage output in an additive manner. If 500 pairs of couples are connected in series, they would therefore produce about 250 mV/.degree.C., or about 250 nV/10.sup.-6 .degree. C., which is sufficient for the needs of calorimetry. For 500 couple pairs, the resistance of each arm must be less than 1,000 ohm in order to keep the total sensor resistance below 1 megohm. A higher resistance would increase measurement noise to undesirable levels and place unusual impedance requirements on the voltage measurement device. These considerations constrain the dimensions and the doping of the individual silicon couples. The calorimeter sample cell size and thermal conductivity requirements set dimensional limits for the total sensor size.
A number of researchers have reported microfabricated thermopiles for purposes other than calorimetry, using single crystal silicon or doped poly crystalline silicon, and supported on thinned silicon substrates (Moser and Baltes, 1993, van Herwarrden et al., 1989) or on quartz (Kiely et al., 1994), or using other materials such as aluminum or gallium arsenide or gallium aluminum arsenide. However, in these reports, there are typically tens of couples with hot to cold separations of around a hundred microns, whereas the subject device incorporates a hundred or more couples with a total sensing separation of millimeters.
In heat conduction calorimetry, a differential temperature sensor measures the temperature difference between the sample container at thermal equilibrium (due to the sample's evolution or absorbance of heat) and the reference heat sink. The sensor signal is proportional to the sample's heat rate. However simple in theory, practical implementations are sophisticated and expensive instruments. The thermal resistance of the connection of the sample cell to the heat sink is necessarily kept low, on the order of 1 watt/degree C., which yields a temperature difference of approximately 10.sup.-6 .degree. C. for each microwatt of heat flux. A higher thermal resistance may be employed, thus providing a larger temperature difference, but the increased signal results in longer equilibration times (Spink and Wadso, 1976). Even with low thermal resistance, the major limiting factor in the rapid measurement of heat rate is the time required for the sample and its container to reach thermal steady state within the instrument. This requires a minimum of fifteen to sixty minutes, depending on the measurement sensitivity, the system thermal dynamics, and the heat capacity and conductivity of sample and container. Since meaningful measurements cannot be acquired before steady state is approached, sample throughput for each cell is severely limited. Significantly higher processing rates can only be achieved by employing multiple sample stations; therefore the cost per station must be kept low if affordable multi-sample instruments are to be produced.
One common current design element that contributes to the high cost of modern, sensitive calorimeters and thus impedes the production of affordable high throughput instruments is the need for expensive or low sensitivity sensors. As indicated above, thermopiles are the thermal sensor of choice, since they are self exciting and therefore introduce no heat into the measurement system. However, instrument manufacturers currently must choose either commercially available Peltier modules intended for cooling applications and having relatively few junctions, or custom constructed arrays of many thermocouple junctions that are prohibitively expensive. A typical microcalorimeter employing a Peltier module with 10 to 100 bismuth-telluride couples would generate only 2 to 20 nanovolts per microwaft of heat rate and thus require special circuitry for reliable measurement. Even this sensor is relatively expensive. Where several sample cells are desired, each must have its own amplifier, since solid state multiplexers cannot be effectively used at these low signal levels.
There have been many reports and patents describing the construction of a variety of thermopiles, including sensors microfabricated from polycrystalline or single crystal silicon. However, all of these reports concern devices created with a sensing area contained within the plane of the sensor surface, with the output proportional to the temperature difference between this area and an adjoining reference area.
Additionally, U.S. Pat. No. 3,071,495 discloses a method of manufacturing a Peltier thermopile. The method comprises a system for vapor-depositing substances to generate the thermopile.
A thermopile, radiometer, and method for producing same are related in U.S. Pat. No. 3,267,727. Quantitative analysis of 2.pi. and 4.pi. radiant environments is accomplished by producing a thermopile panel having a matrix of thermoelectric junctions.
Described in U.S. Pat. No. 4,111,717 is a thermopile utilized in radiation pyrometry. Thermopile leads are evaporated onto a thin substrate together with a pattern distribution of thermocouple junctions. Included are relatively large reflective areas associated with the region of the thermopile where the cold junctions are located.
U.S. Pat. No. 4,343,960 communicates a thermopile and manufacturing process. Plating and photo-etching techniques are employed to produce a thermopile having segments of one metal and segments of another metal, and the segments of different metals are connected to one another alternately so that the thermocouples are arranged in series on a heat-resistant electrically non-conductive substrate. Each segment has a portion plated on one surface of the substrate and a portion plated on the inner wall of through holes formed in the substrate to connect the two portions plated on the two surfaces of the substrate with each other.
A fully integrated single-crystal silicon-on-insulator process, sensors and circuits are disclosed in U.S. Pat. No. 5,343,064. A layered system is related that includes all of the required components for sensors like capacitive accelerometers and pressure devices.
U.S. Pat. No. 5,059,543 presents a method of manufacturing a thermopile infrared detector. A doped semiconductor supporting rim supports a series of polycrystalline silicon and metal thermocouples. The fully doped semiconductor area serves as an etch stop for a single-sided etch which eliminates the need for front-to-back alignment of the device.
U.S. Pat. No. 5,434,744 reports a method of manufacturing a thermoelectric module that has reduced spacing between semiconductor elements. Alternating bars of thermoelectric materials are arranged between two electrically conductive patterns on two opposing substrates.
A high-sensitivity, silicon-based, microcalorimetric gas sensor is characterized in U.S. Pat. No. 5,451,371. A gas particle sensor for an internal combustion engine includes a pair of polysilicon plates, each plate supporting a pair of resistors, one serving as a heater and the other as a thermometer, one plate being coated with a catalyst to promote combustion of unburned combustible gas constituents.
For the subject invention, a microfabricated thermopile is optimized as a differential temperature sensor. Heat conduction calorimetry is based on the measurement of the temperature difference between a sample container and a stable thermal reference. The subject sensor provides increased sensitivity over the prior art at a lower cost than present sensors, thus allowing the more economical manufacture of such items as a multiple channel calorimeter and in other differential temperature measurement applications. Also, the increased sensitivity of the subject sensor over the prior art permits the use of off-the-shelf data acquisition hardware, further lowering the cost of design and manufacture for instruments utilizing the sensor.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully submitted, however, that none of these patents teach or render obvious, singly or when considered in combination, applicant's claimed invention.